Light-scattering type smoke detectors and ionization type smoke detectors are known. The respective detectors are of different sensitivity to smoke, which makes it desirable to use these two detectors in combination in order to detect smokes of different constituents.
In known combination type smoke detectors, a light-scattering smoke detector having a smoke detection chamber, and, an ionization smoke detector having a separate smoke detection chamber, are provided in a single housing. This combination type of smoke detector inevitably is of large size.
In a light-scattering type smoke detector, it is essential to prevent unwanted light from reaching the light-sensing element as this produces noise in the wanted signals. Therefore, careful consideration is paid in order to insure that the inside wall and members and parts in the dark chamber do not reflect light.
In the light-scattering type smoke detector, the inside surface of the dark chamber preferably is finished at fineness of 800 mesh or more so as to prevent diffused reflection rather than giving it a light-absorbing finish, such as matted finish or lusterless black coating. The most desirable light-scattering smoke sensor known so far is provided with fine inside finish and a conical projection on the bottom plate in order to deflect the reflected light away from the direction of the light-sensing element. In cases where the dark chamber is sufficiently tall, the reflecting cone is unnecessary.
A most efficacious light-scattering smoke detector is one provided with a dark chamber of so-called labyrinth structure as disclosed in U.S. Pat. No. 4,216,377, Hasegawa. The labyrinth dark chamber comprises a circular bottom plate and a lateral wall which is formed by a plurality of angled lamina members which allow passage of smoke into the chamber while preventing the admission of light. Each lamina member has a cross section comprising a first straight or slightly arcuate portion along the circumference of the bottom plate of the dark chamber, a second straight portion extending from one end of the first portion at an obtuse angle, and a third portion extending from the free end of the second portion and towards an adjacent first portion. The lateral wall of the dark chamber is formed by arranging a plurality of such laminae in overlapping relation with serpentine paths left therebetween. Such a complicated structure cannot be economically manufactured of a metallic material, and labyrinth dark chambers are usually made by molding a plastics material.